Trevor I am partly convinced by X factor in some respects but have significant doubts over its use in practical fashion. Interesting that you mentioned Tar Heel/Bret Hanover in X factor positions as they do provide another significant x factor presumed trace to Oniska. Helen Hanover and Margeret Parish were the mares I was thinking of so that actually gives SBSW traces to 3 presumed significant mares.

Autopsy results on TB's at least do confirm the existence of individuals with large healthy hearts, as opposed to unhealthy enlarged hearts, which obviously must have occured as a result of their genetic heritage. Science has confirmed that the X chromosone does cary more genetic material then the Y and that certain traits can be sex linked, which could include the large heart gene. Practically speaking though in our herd today X factor traces are extremely prolific and champions of the track and breeding shed still comparatively rare so its hard to fathom the significance of X factor alone in the production of champions, though you could argue it could have influenced the overall improvement of the breed. Imo if an x factor as such exists it probably needs the presence of additional genes or gene interaction to bring it to the fore and into play as a component of the make up of a superior horse.